The use of spark gap switches for applications requiring very high operating voltages and currents such as pumping pulsed gas discharge lasers, is well known. A conventional spark gap switch includes a pair of electrodes spaced far enough apart such that a voltage applied across them is insufficient to bridge the gap between them until triggered. This type of switch is an excellent insulator for voltages below the hold-off value or breakdown voltage of the gap, providing a high degree of safety.
When current flow is desired across the gap, the gas between the electrodes (usually air) must be sufficiently ionized to cause the gap to break down. This may be accomplished by a sudden increase of voltage across the gap, a sudden reduction in density of gas dielectric between the electrodes, natural radio active irradiation of the gap, ultra violet irradiation of the gap, a heated filament in the gas dielectric around the gap, distortion of the electric field formed in the gap, or injection of ions and/or electrons into the gap. As is well known in the art, all of these methods require substantial and often cumbersome triggering mechanism.
One technique for triggering the breakdown of the gap between electrodes is the use of a mid-plane or triggering electrode placed in the gap between primary electrodes as shown in FIG. 1. The gap between primary electrodes 11, 12 is approximately cut in half by the positioning of mid-plane electrode 13. Typically such a spark gap switch is exposed to high pressure (1 atmosphere or greater) around its electrodes. The hold-off voltage of the switch is determined by the electrode spacing (gap) and the gas pressure between the electrodes. When current flow between the primary electrodes 11, 12 is desired, a trigger voltage (usually a high voltage pulse) is applied, as shown, to the mid-plane electrode 13. The high voltage trigger pulse causes localized ionization between the edges of mid-plane electrode. If the electric field across the primary electrodes is sufficiently high the ionization spreads throughout the gap between the primary electrodes. As a result of the spreading gas ionization, the gap breaks down and current flow between the primary electrodes is initiated. A spark gap switch using a triggering electrode is found in U.S. Pat. No. 4,604,554 issued to Wooten on Aug. 5, 1986 and entitled, "TRIGGERED SPARK GAP DISCHARGER."
Generally, the circuit providing the high voltage trigger pulse to the mid-plane electrode includes a power supply, a pulse transformer, capacitors and other appropriate components. The trigger circuit operates at high voltage and is consequently expensive. The high voltage components of the trigger circuit also introduce a time delay into the operation of the spark gap switch, making rapid triggering and precise timing in systems using such switches problematical.
The problem of imprecise timing becomes more critical in systems using multiple spark gap arrangements such as a Marx generator, which requires simultaneous closing of all its spark gap switches for optimal operation (FIG. 3). Further, each spark gap switch has its own high voltage triggering circuit and power supply. The power supplies, sometimes not effectively isolated from each other, often are required to "float" above ground potential. These conditions can be dangerous as well as further complicating the timing of the spark gap switches.